Pregnant women exposed to higher levels of the chemical bisphenol A gave birth to baby boys with lower thyroid hormones, according to a new study published today.

The study by University of California, Berkeley, scientists is the first to link the ubiquitous chemical – used in hard plastics, canned food liners and some paper receipts – to altered thyroid hormones in babies, and it adds to evidence that BPA may have some effects on fetuses.

For every doubling of the mothers’ BPA levels, there was 9.9 percent less thyroid-stimulating hormone in their baby boys. No significant effect was detected in the girls; animal studies suggest females may be able to metabolize the chemical better.

“Most work up to this point has focused on [BPA’s] estrogen properties. The fact that it’s also messing up thyroid function is very surprising,” said Laura Vandenberg, a postdoctoral fellow at Tufts University who studies BPA but was not involved in the new study.

Experts said they do not know what, if anything, the reductions in thyroid hormones might mean for the health of the babies because their levels remained within the range considered normal. But previous research suggests that reduced thyroid hormones might impair learning abilities and motor skills.

“Thyroid hormone is probably the best known factor in terms of influencing brain development,” said Thomas Zoeller, a biology professor at University of Massachusetts-Amherst who specializes in studying thyroid hormones and brain development. “The fact that the researchers see relationships at all is a very important issue that we should not ignore."

The scientists analyzed data from 364 pairs of moms and newborns in California’s Salinas Valley, a low-income community of mostly Mexican-American farm workers. Most of the mothers had low levels of BPA – 42 percent less than the average U.S. woman. Eighty-two percent had the chemical in their urine, compared with 95 percent of women of childbearing age tested nationwide.

While the study does not prove that BPA alters babies' thyroid hormones, scientists say it provides evidence that the link should be further investigated.

“Our data suggest that there is not a safe level of exposure,” said Jonathan Chevrier, an assistant researcher at the University of California, Berkeley, Center for Environmental Research and Children's Health and lead author of the study, which was published in Environmental Health Perspectives.

The American Chemistry Council, which represents chemical manufacturers, maintains that BPA is safe to use and questioned the validity of the study.

"The author's speculation that BPA is linked to health effects caused by thyroid hormone levels in women and newborns is not supported by the data," said Steven Hentges, of the council's Polycarbonate/BPA Global Group, in a prepared statement. "The authors themselves note that the thyroid hormone levels reported were within normal range and the study was not designed to measure any health effects."

He also said the testing was too limited to be an accurate representation of the mother's levels during pregnancy.

Thyroid hormones are continually produced, and BPA doesn’t stay in people’s bodies for long, but Chevrier said this doesn’t mean the findings aren’t cause for concern.

“This effect we see might not be permanent…Some people might think that’s good news,” he said. “But people are continually exposed to BPA. The effect isn’t permanent, but people are permanently exposed.”

However, Zoeller cautioned that "because BPA is not bioaccumulative, it is very difficult to get a good sense of what exposure is."

"Even with multiple measurements, you have to start out realizing that establishing these links is enormously challenging," he said.

Results from previous studies investigating BPA and thyroid function have been inconsistent. Different studies on rats have shown BPA to increase, decrease and have no effect on thyroid hormones after prenatal exposure. In human studies – before now only conducted on adults and teens – BPA has been linked to both increased and decreased thyroid hormones.

The new study found significant decreases in thyroid hormones in the babies' blood only when compared with the women’s BPA levels shortly before they gave birth, not during their first and second trimesters. That may mean that BPA’s effect on the fetuses’ hormones is temporary or comes only at certain times.

“This association was strongest when BPA was measured in the third trimester of pregnancy, which may either be due to a transient effect of BPA on thyroid-stimulating hormone or a developmental window of susceptibility,” the study said.

The mothers also had reductions in some thyroid hormones that correlated to their BPA levels. However, the type of hormone, called T4, "is not biologically active" so the importance is unknown, the researchers wrote. The study controlled for their iodine intake, which can affect thyroid hormone levels, and for other chemicals they were exposed to.

The researchers, led by epidemiologists Brenda Eskenazi and Kim Harley, have tracked this group of Salinas Valley women and their children since before they were born in 1999 and 2000. They are investigating an array of chemicals, particularly pesticides, to investigate whether the children’s health is at risk.

The children tested as newborns for thyroid hormones in this study are now teenagers, and Chevrier and his colleagues plan to look for any potential effects of the lower hormones. "We are aware of no studies that investigated the developmental effect of lower but normal neonatal TSH," they wrote.

Only a few studies have investigated the possible neurological effects of BPA in children, including two that found a link to hyperactivity in girls and other behavior problems. Animal studies have found impaired memory, gene changes and altered behavior.

Cheryl Stein, an assistant professor at the Mount Sinai School of Medicine, said usually it is high thyroid hormone levels that are cause for concern because they can spur development problems or mental retardation. This is because higher levels indicate the body is trying to make up for a thyroid that isn’t functioning properly.

Scientists in the Netherlands studied 220 women and reported in 1999 that those with the lowest thyroid hormone levels had babies who scored lower on mental and motor skill tests. They warned that it “may be an important risk factor for impaired infant development.”

Zoeller, who was not involved with the new study, said the potential health impacts remain vague because it measured hormones in the babies’ blood instead of tissue, which gives a more complete picture of their functioning.

“This should really be a trigger to look further into this relationship,” Zoeller said.

In animals, some studies report that low levels cause reproductive problems, obesity and cancers. In human adults, it has been linked to increased risk of diabetes and heart disease.

The idea that “many lifelong chronic diseases start in the womb” has gained wide support in recent years, according to a 2012 study from the Center for Research on Occupational and Environmental Toxicology. And “pretty much anything mom is exposed to the fetus is exposed to,” Vandenberg said.

BPA circulates in pregnant women’s blood, and it passes through the placenta the same way alcohol would, she said. It’s also found in amniotic fluid, which cushions fetuses in the womb and provides nutrients for development.

BPA is mass-produced, heavily studied and controversial. Each year about six billion pounds are produced globally and more than one million pounds are released into the environment, according to the U.S. Environmental Protection Agency.

A synthetic estrogen, BPA has been used as an ingredient in polycarbonate plastics since the 1950s. It also is used to produce resins for the liners of canned foods.

Since BPA is ubiquitous, it’s hard to pin down how people are exposed, but Vandenberg said canned food is a major source.

Currently, 11 U.S. states have banned BPA in some products. In July, the U.S. Food and Drug Administration banned the chemical from baby bottles and sippy cups after manufacturers had already abandoned its use in these products. Earlier this year, the FDA denied a request to ban uses in food packaging but announced that it was "not a final safety determination" because research continues.

This article originally ran at Environmental Health News, a news source published by Environmental Health Sciences, a nonprofit media company.